1. Field of the Invention
The invention pertains to a thermal detector, particularly a thermistor bolometer operating in the infrared domain, wherein modulation of the output is achieved electronically rather than mechanically.
2. Description of the Prior Art
In the prior art, the use of thermal detectors, such as thermistor bolometers, is well-known. Common applications are satellite guidance systems such as disclosed in patent application Ser. Nos. 08/227,270 (U.S. Pat. No. 5,455,429) entitled "Earth Sensor for Satellites" and 08/229,460 (U.S. Pat. No. 5,477,052) entitled "Earth Sensor for Satellites with Radiance Compensation", or detection of overheated railroad bearings such as disclosed in U.S. Pat. Nos. 5,100,243 and 5,331,311, all commonly-owned with the present application.
Thermal detectors, particularly thermal bolometers working in the infrared domain, are used in two modes, either staring or chopped. Each mode has its strong points and limitations. The chopped mode requires a mechanical chopper to periodically interrupt the incident radiation. The chopper, a mechanical device, is subject to failure. The chopped mode provides an AC output which simplifies the electronics in terms of noise control and signal amplification. The staring mode does not employ a chopper but requires greater care in signal processing because it provides a d.c. signal.